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Name: Yvonne
Status: other
Grade: other
Location: CO
Date: November 2007

How long does it take to get the chlorine to evaporate out of a gallon of water?

Hi Yvonne -

That's a tough question, because it depends on a few things. First off, how much chlorine is in the gallon of water (in other words, what is the concentration?)? What is the temperature of the water? What is the temperature and humidity around the bucket? What is the rate of air movement around the bucket? So to answer the question, we'd need to know a bit more, most importantly, the concentration of chlorine in the bucket. Are you just thinking of swimming pool water?

Do let us know so we can get you a better answer!! :)


Susannah Sample
University of Wisconsin
School of Vet. Med.


Filtered water will not have chlorine in it and tap water will not harm most non-aquatic animals. However, for fish, it is best to let the water sit over night and the chlorine will have evaporated to save levels. There are products in pet stores that will eliminate chlorine quickly by adding it to your water.

The concentration of chlorine in tap water is relatively low in the first place, and even with fish, the reason you want to eliminate the chlorine is due to the chemical reaction between the chlorine and the fish excretions that react to produce a harmful complex that will kill the fish.



This may not be a simple question. It could be minutes or years. I think it depends on so many details:

How much exposed surface area on your 1.00 gallon?
(A tall thin glass cylinder will be slower than a broad shallow tray)

How high is the temperature?
(Like pool chlorine, goes faster when it's warmer.)

How fast is the air going by?
(Very stagnant air should slow things down. Very fast air brings in assorted trace substances faster: dust, CO2, ammonia, maybe reducing agents and other things.)

Is it pure inert gas, clean air, or some random dirty enviroment?
(Pure N2 makes a simpler, more repatable, better-defined science experiment. Clean filtered air still usually contains CO2, which can add acid. Dirty air can contain an unknowable number of other things that can matter. )

What impurities that influence pH are drifting by in the air?
(how much CO2; is there ammonia or ammines?)

Is the water circulating or stagnant?
(Deep and stagnant could take longer.)

What is the pH of the water?
(being alkaline or basic tends to lock the chlorine in; being acidic exposes the chlorine and allows it to evaporate. Being very basic can even change chlorine to chlorates.)

What formula and concentration of chlorine are you refering to in your phrase "the chlorine"?

All this stinks, I know.

If pure water with some air-space over it is in a closed inert conatianer with some nitrogen gas coming in thru one tube, bubbling up thru the water, and exiting thru another tube at a known volume flow rate, then it could be precisely calculated how fast the chlorine evaporates using the equilibrium formula.

Given the concentration of dissociated chlorine in the water, the equilibrium formula is inverted to dictate the density of Cl2 in the bubbles, and that density times the nitrogen's volume flow rate becomes the amount of Cl2 removed from the solution per unit time. Unfortunately that's about the cleanest this question can be.

And it still depends on temperture, gas flow rate, and pH. You could pick one value for each. pH being something acidic.

Forms of chlorine:

Pure Cl2 gas bubbled into water H2O dissociates to make HOCl + HCL in the water.

Both H's become acidic. (OCl)- is called "hypochlorite ion". It's an equilibrium; the reaction goes either way easily. The concentrations obey an equilibrium formula such as:

k(eq) = [HOCL(aq)]*[HCl(aq)] / [Cl2(gas)].

Changing temperature changes the value of k(eq). pH affects the percentage of OCL- that "counts", that which itself as HOCL molecules. (Affects HCL too, I suppose.) Circulations affect the rate Cl2 gas is flushed out of the area.

Some people saying chlorine might mean old-style swinning-pool chlorine, which is NaOCl solution. NaOCl is strongly basic, so pool acids are added to the water to bring it back to neutral pH=7. Laundry bleach is a very similar NaOCl solution.

New-style "stabilized" pool chlorine, as fresh powder in it's jar. is a 6-member ring molecule made of alternating N atoms and C=O ketone groups. To each N (or sometimes to each O) is attached a Cl atom or an Na (sodium) atom. With 3 Na's it's just called "chlorination stabilizer". With 2 or 3 Cl's it's called "stabilized chlorine". In the water, chlorine attached to the ring molecules is not availble for evaporation, and also not speedily attacking life-forms, so you can store more in the water without smelling so much chlorine it makes your eyes and lungs hurt. Then it takes longer for evaporation to get rid of it all, easier for the maintainer. This is a complication you don't want to include in your question.

The "chlorine" in tap water can be Cl2 or NaOCl when it's at the water company, but I've heard they eliminate most of that before it's sent down the pipes to you. A milder residual form such as "chloramines" is likely to have higher concentration in your tap water when you draw it. Chloramines might need a different pH (basic?) to evaporate fast.

The OTO drops test used for pool chlorine, it might have similar sensitivity to all these types, I'm not sure.. That would help you out doing an experiment, by making the form of chlorine less important. You should still try to control the many variables I mentioned: temp, circulation, pH, clean gas.

Boiling away some percentage of the water (10%?) probably gets rid of low concnetrations of any form of chlorine. Don't try to boil pure pool chlorine or bleach; if that much chlorine gas comes out it will be dangerous. Diluted to 0.1% in water is about the strongest that's OK. A few drops per gallon is the usual recipe for chlorine-preserved water. That's less than 0.01%.

I suspect you'll need to learn things and pin down your question, to answer it.


Jim Swenson

Asking "how long" or "how fast" chemical or other processes occur frames the question in a way that makes it impossible to answer. The "rate" that is the "how long" or "how fast" depends upon a lot of variables that are difficult to control. In the case of your question, a partial list of questions would be:

What is starting concentration? What is the pH? What is the temperature? Do you mean chlorine gas, or do you mean commercial "chlorine bleach"? What are the other components in the solution?

Until all of these constraints ( and other) are specified you can get most any answer you want.

Be very cautious about "how fast" and "how long" questions. It is possible to address what is the composition of the gases in equilibrium with a certain set of compositions in the liquid phase. But to address the "how fast" or "how long" requires a lot more data setting the constraints.

Vince Calder

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